Motors, engines, pumps and like rotary apparatus directly or indirectly use a rotating shaft as the driving member to turn rotors, gears, pulleys, cams and the like driven members. Unless formed integrally in one piece with the shaft, such driven members are locked to the shaft with a coupling, such as a key and slot arrangement. Such a coupling arrangement allows for replacement of gears, cams and other driven members linked to the rotatable shaft by simply removing the key and withdrawing the driven member. It also permits accurate alignment of one member relative to the other with substantially no looseness or play therebetween.
The key, an elongate rectangular member extends into a complementary rectangular shaped key way or slot located, respectively, on each of the shaft and driven member, bridges the two and thereby rigidly joins such parts together for joint rotation. Typically such keys are formed of hardened steel. In transmitting the rotational force or torque from the shaft to the member that force often is becomes concentrated at the top edge or corner of the key, as when the slot widens due to wear, and focuses stress on portions of the driven member.
The existence of right angled corners in the key ways does not result in abnormal stress on materials of great tensile strength and, in most instances, the normal operations do not create sufficient forces to stress those materials to the point of cracking at the corners. As example, the gears in gear trains are usually made of the same material as the key and the shaft so that the stresses of starting, accelerating and stopping do not place abnormal burdens on the coupling. However, in materials of relatively low tensile strength, the creation of stress cracks at the corners of the rectangular shaped key ways produced by a hard steel key can result in equipment failures even under normal operation.
In some rotating mechanisms the gear or rotor may be formed of a relatively softer or more malleable material than the driving shaft. The use of a rectangular steel key in a rectangular groove in that application could result eventually in cracking or fractures of the gear or rotor under the stresses from starting, accelerating and/or stopping of the shaft.
As example, modern high performance automobiles, such as those used in automobile racing, incorporate dry sump oil pump systems to provide engine lubrication under the extreme operating conditions encountered in automobile racing, such as the oil pump systems manufactured since the `70`s by Cosworth Company of the United Kingdom. An oil pump system of that type, in addition to the principal oil pump, contains one, two, three or more scavenger pumps, the number being dependent upon performance needs, which scavenge oil from the engine and return it to the oil sump. Each scavenger pump employs a pair of rotors of a particular shape, referred to as the Roots rotors, arranged for counter rotational movement driven by a pair of steel shafts.
With so many rotors in such an oil pump system in order to keep the weight down and for ease of manufacturing to close tolerances, which are important considerations for racing automobiles, the pump rotors are formed of Aluminum, which is softer than steel. The pump's driving shafts, however, require greater strength and thus continue to be formed of steel, a tougher heavier metal than Aluminum. Hence the key coupling in that oil pump system must join members having such dissimilar physical characteristics, such as hardness, and still avoid creation of debilitating stresses on the rotor. The key coupling of the present invention as an advantage achieves that result.
Further, assembly of the foregoing oil pump requires accurate placement of the rotors between the side walls of the compartment associated with a particular scavenger pump. The shafts and rotors and the housing parts, such as the compartment walls and spacers separating those walls are machined to very tight tolerances. However, even with such tight manufacturing tolerances, slight misalignment between the compartment walls and the rotors could still result, whereby the flat end of a rotor rubs against the wall or may bind. One way to adjust that relationship is to allow the rotor to be axially adjustable in position on the shaft or "float" but such a feature is not believed to be compatible with the existing key coupling arrangement.
With the tight fit with which the rectangular key fits into the slot, the rotor cannot be adjusted with the key in place. If the key slot is designed to be wider so the key fits loose in the slot, instead of the customary tight fitting relationship, the force transmitted through the key becomes more focused at the upper edge of the key and increases stress on the softer aluminum rotor as could cause premature failure. The tight interconnection typically required with the rectangular key coupling between the shaft and the rotor thus is not the preferred or desired mode of operation in such an oil pump system and may not likely be preferred in other types of rotary devices as well. As an advantage, the improved key coupling of the present invention permits the rotor in such application to float.
Accordingly, an object of the present invention is to provide an improved means to couple a driving member and a driven member for joint rotation and to provide a novel method for producing such coupling.
Another object of the invention is to improve the manufacturing efficiency, reliability and effectiveness of a dry sump type automotive oil pump system, particularly those oil pump systems which incorporate rotors formed of light weight metal rotors, such as aluminum, and driving shafts formed of harder metals, such as steel.
A further object of invention to provide a novel coupling of driving and driven members formed of dissimilar materials having dissimilar tensile strengths that minimizes stress induced cracks or failures of such members.
An additional object of invention is to provide an improved key coupling system for coupling rotors formed of one material to a shaft of dissimilar material that avoids the use of sharp corners in the key.
It is a still additional object of the invention to provide a coupling for rotary apparatus that allows the driven member to float on the shaft.